1. Field of the Invention
This invention relates to electro-chromic displays (ECD) utilizing the electro-chromic phenomenon, i.e., variation in the light absorption characteristics by application of appropriately controlled voltages, and more particularly, to an improvement in the sealing compound for the display cells.
2. Description of the Prior Art
Electro-chromic displays are roughly classified into two types. According to one type, the electrochromic material in a solution undergoes an electrochemical redox reaction, and forms a colored, insoluble reaction product on a cathode. The other type utilizes a thin film of an inorganic solid, such as amorphous tungsten oxide (WO.sub.3), with an electrolyte such as sulfuric acid. The amorphous tungsten oxide develops a blue color if a transparent electrode coated therewith has a negative potential against the opposite electrode. Both types of electro-chromic displays are bleached of their colors if the polarity of the voltage applied thereto is reversed.
The electro-chromic display has the following features:
(1) It has a wide view angle;
(2) It is possible to develop several different colors;
(3) It consumes an amount of energy which is proportional to the number of coloration and discoloration cycles, and which is from several to several tens of mJ/cm per cycle; and
(4) It has a memory function, i.e., it can maintain the colored state for several hours or days after removal of the coloration voltage if it is kept electrically open. The memory does not require application of any electric power from an external source.
The application of the electro-chromic displays having the aforementioned features for various kinds of display apparatus is of great interest.
Reference is made to FIG. 1 showing an electrochromic display cell by way of example in cross section. A dish-shaped substrate 1 is made of glass, ceramic or like material. An counterelectrode 2 and a reference electrode 3 are formed inside the substrate 1. A transparent substrate 4 positioned on the display side of the cell carries thereon a transparent electrode 5 formed from indium oxide (In.sub.2 O.sub.3), tin oxide (SnO.sub.2) or the like, and a display electrode 6 formed from tungsten oxide (WO.sub.3) or the like, which are both formed by vacuum evaporation. The transparent substrate 4 is usually made of glass. A white porous plate 8 is sandwiched between layers of glass fibers 7, and disposed between the counterelectrode 2 and the reference electrode 3, and the display electrode 6. The white porous plate 8 is provided for establishing a white background, but it is possible to employ a porous plate having a color which is other than white. The substrates 1 and 4 are sealingly joined to each other with an adhesive 9 to define a display cell. An electrolyte 10 is introduced into the cell through an inlet provided in the substrate 1, but not shown. The inlet is tightly closed after introduction of the electrolyte. The electrolyte 10 may comprise a cellosolve acetate or .gamma.-butyrolactone solution of lithium perchlorate having a concentration of 1 mol per liter. A mass of bubbles 11 is provided for absorbing the difference in the degree of cubic expansion rate between the substrates 1 and 4, and the electrolyte 10.
An addition reaction type silicone adhesive, as disclosed, for example, in Japanese Laid-Open Patent Specification No. 61951/1979, has hitherto been used for sealing an electro-chromic display cell of the type hereinabove described. This adhesive comprises a silicone polymer which is addition polymerized when heated in the presence of a catalyst. It is usually a two-part adhesive which is cured in rubber or resin form by forming dimethylene bridges in accordance with the following formula: ##STR1##
The principal features of this adhesive are as follows:
(1) It is a self-tack type adhesive which eliminates the necessity for any special pretreatment of the glass substrate;
(2) It is a solventless, addition reaction type adhesive which does not form any reaction by-product that may contaminate the electrodes in the ECD cell; and
(3) As it requires a long time for curing, it has a long pot life.
Although the addition reaction type silicone adhesive has a number of excellent features as hereinabove set forth, the electrolyte in an ECD cell sealed with such a silicone adhesive leaks out in gaseous form through the adhesive layer and is lost, if the cell is maintained at a high temperature. This is an important cause for reduction in the reliability of the cell.
In view of the foregoing problems, it is an object of this invention to provide a reliable ECD cell in which no decrease occurs to the electrolyte.